1. Field of the Invention
This invention relates generally to the production of metallic oxide compounds having a fine particle size. More particularly, the present invention pertains to the production of metal oxides having a spinel-type structure by simple chemical precipitation and conversion reactions. Specifically, such reactions involve the conversion of mixtures of particulate hydrated oxides of iron with effective metallic solutions under various operating parameters of temperature, time of reaction, concentration of reactants and pH effects. The conversion reactions may also provide for the insertion of desired metal ions into precursor metal oxide crystals to produce crystalline products having compositional and physical characteristics deemed highly desirable and useful in diverse areas of commerce.
2. Description of the Prior Art
The prior art is replete with methods for converting or transforming the various oxides of iron into any of numerous forms. One form of an oxide of iron, magnetite (Fe.sub.3 O.sub.4 =FeO.Fe.sub.2 O.sub.3), is presently produced by prior art processes which are both uneconomical and cumbersome. For example, Frey U.S. Pat. No. 3,036,889 teaches a method for producing the black oxide of iron, or magnetite, having a fine particle size and suitable for pigments, by reducing Fe.sub.2 O.sub.3 with hydrogen at elevated temperatures according to the following reaction: EQU 3Fe.sub.2 O.sub.3 + H.sub.2 .fwdarw. 2Fe.sub.3 O.sub.4 + H.sub.2 O
another known method for making Fe.sub.3 O.sub.4 involves the simultaneous oxidation and neutralization of a waste pickle liquor solution to precipitate Fe.sub.3 O.sub.4. Taylor et al U.S. Pat. No. 3,434,797 discloses such a process while utilizing a waste hydrochloric acid pickle liquor containing 20% to 30% by weight of FeCl.sub.2. Similarly, Rathmell U.S. Pat. No. 3,261,665 discloses the same basic process while utilizing a waste sulfuric acid pickle liquor containing FeSO.sub.4. The type of precipitated Fe.sub.3 O.sub.4 derived by this basic method may be employed in magnetic inks, in "ferrofluids" for magnetic seals or clutch devices, or any other application where a finely divided magnetite product is required.
The prior art recognizes various methods for producing complex ferrous-metallic compounds which are useful in various, diverse technologies. Along with magnetite, such metal oxide compositions are classified as ferrites. Exemplary of such ferrite formulations are Zn.sub.x Fe.sub.y O.sub.z and Ni.sub.x Fe.sub.y O.sub.z. However, as with known methods for producing magnetite, the prior art procedures for making ferrites are also possessed of undesirable characteristics. For example, a known process for producing ferrites, exemplified by Kenney et al U.S. Pat. No. 3,129,184, discloses ferrites having improved magnetic properties which may be made by mixing iron oxide particles with an oxygen-containing derivative of a divalent metal, such as zinc or nickel carbonate, oxide or hydroxide. The mixture is calcined to drive off carbon dioxide, and to cause a reaction between the iron oxide and the oxide of the divalent metal. The resulting ferrite crystals may be ground to finer sizes and recalcined in order to stabilize their magnetic properties and prepare then for ultimate use in sintered, compact form for applications in the electrical or electronics industry. Large quantities of ferrites produced by this basic process are used where "soft" magnetic properties are required.
Another known procedure for making soft ferrites is exemplified by the Albers-Schoenberg U.S. Pat. No. 3,019,189. This method is based upon the co-precipitation of two or more metal oxide hydrates from salt solutions of the metals. The mixed hydrates are separated from the solution, washed, dried and calcined to obtain ferrite crystals of the desired composition. Co-precipitation is characterized by a very intimate mixing of the raw materials and, thus, ferrite cores prepared from co-precipitated products are believed to be superior in quality to cores made from mixed oxide constituents.
All of the aforementioned prior art methods, and variations thereof, for making magnetite and its related, complex compounds are hereby rendered obsolete by the process of the present invention which is based upon the use of hydrated crystalline iron oxide in one of its lesser known forms, beta FeOOH.
There are four modifications of hydrated iron oxide or oxyhydrates that are presently known. These are alpha, beta, gamma, and delta FeOOH, of which alpha FeOOH is the most widely distributed in nature and which has been more fully studied and documented than the other forms. Upon dehydration, all hydrated iron oxides transform to hematite (Fe.sub.2 O.sub.3), but this transformation may be different for the various forms, and hematite obtained therefrom may have differences with respect to shape and size of the particles, magnetic properties, morphologic characteristics and color variations.
Of the various forms of hydrated iron oxide, the most exotic is the beta form. Written as beta FeOOH = beta 1/2 Fe.sub.2 O.sub.3.H.sub.2 O, this hydrated iron oxide form is thought to be tetragonal with lattice parameters of a = 10.48A and c = 3.023A, and having the structure of hollandite. Sub-units, each a/3 .times. a/3 .times. c, containing two oxygen atoms in an approximately body-centered cubic arrangement, can be distinguished with Fe.sup.+++ ions lying in octahedral interstices. This form of the oxide is unique in that it appears to be stable only when it contains chloride ions, with a washed precipitate of the beta FeOOH normally containing from two to four percent chlorine.
The beta form of hydrated iron oxide occurs as the mineral "acaganite". This form of the oxide can also be found in small amounts in the corrosion products of some steels. However, the two largest potential sources of beta FeOOH are from the regeneration of spent hydrochloric acid pickle liquor solutions and the regeneration of FeCl.sub.3 leaching solutions used in metallurgical separation reactions. Both types of solution can be processed to yield a precipitate of beta FeOOH in the form of needle-like particles.
The normal conversion to the beta form of the hydrated iron oxide takes place in the following oxidation-reduction reaction: EQU 6FeCl.sub.2 + 3/2 O.sub.2 + H.sub.2 O .fwdarw. 4FeCl.sub.3 + 2 beta FeOOH
normally, such a process yields pariculate beta FeOOH of a particle size in the range of from about 0.05 to about 0.009 microns. As the input material for the process of the present invention, this particle size range is usually sufficient since the reaction of the present invention will be operable with an input material of FeOOH in this particle size range.